1. Field of the Invention
This invention relates to a mirror device in a single lens reflex camera of the TTL metering type which permits interchange of the lens, and more particularly to a mirror device for use with such reflex camera in which the area of a portion of the mirror is translucent and a light receiving element is disposed behind the translucent area of the mirror so that the light passed from the picture-taking lens through the translucent area of the mirror is received by the light receiving element.
2. Description of the Prior Art
The most popular cameras of the described type are such that during metering, the light receiving element lies in the picture-taking light path and the light from the picture-taking lens is reflected toward the viewfinder by the mirror while part of the light passes through the translucent area of the mirror to the light receiving element, and that during photography, the translucent area of the mirror now moved up is covered by another member while the light receiving element is retracted from the picture-taking light path.
The cameras of such metering type usually have the following two disadvantages. As a first disadvantage, it may be mentioned that even with the same lens, if the aperture value thereof is varied, the output of the exposure meter does not exhibit a variation corresponding to the nominal aperture value thus changed. The reason is that the outline of the translucent area of the mirror acts as a kind of aperture for limiting the light entering the light receiving element and therefore, the intensity of illumination on the light receiving surface of the light receiving element is not always proportional to the area of the exit pupil of the lens. More specifically, in FIG. 1 of the accompanying drawings, the light ray shown as passing from the exit pupil 1a of a lens 1 (having a nominal aperture value of f/1.4) to the center 2' of the picture plane of a film 2 passes through a rectangular translucent area 4 provided in a mirror 3 and through the opening 5a of a mirror supporting member 5 and enters a light receiving element 6.
The mirror 3, only in the translucent area 4 thereof, causes the light from the lens 1 to pass toward the light receiving element 6 and the other area of the mirror reflects the light toward the unshown viewfinder of the camera. Accordingly, the light rays from exit pupils 1a and 1b are considerably reflected toward the viewfinder, so that the light rays passed through the translucent area 4 are considerably decreased. In contrast, the light rays from exit pupils 1c and 1d all enter the translucent area 4. Therefore, as regards the exit pupils 1a and 1b, the quantity of light from these exit pupils received by the light receiving element 6 is decreased as compared with the quantity of light as determined by their nominal aperture values f/1.4 and f/2. Especially, the decrease in quantity of light is greatest in the case of the light ray from the exit pupil 1a. In contrast, as regards the exit pupils 1c and 1d, the quantity of light from these exit pupils received by the light receiving element is just in accord with the quantity of light as determined by their nominal aperture values f/2.8 and f/4. Thus, the relation between the nominal aperture values of the lens and the output of the light receiving element, namely, the output of the exposure meter, depicts a curve as indicated by a in FIG. 2 of the drawings, which means that this relation is disproportionate. For example, if the aperture value is varied over one or two nominal steps from a nominal aperture value f/2.8 to f/2 or f/1.4, the output of the exposure meter is not equally varied, say, not to two or four times.
If photography is carried out in accordance with such an indication by the exposure meter, the resultant photographs will be irregular in exposure because, in any of the open metering and the stopped-down metering conditions, the indication by the exposure meter will be varied by the aperture value used.
A second disadvantage occurs when the light receiving element is not disposed at the point of intersection of the film surface with the optic axis of the picture-taking lens or at a location conjugate with such point of intersection. That is, even with interchangeable lenses having the same aperture value, if the focal length thereof is varied, or in other words, if the distance from the exit pupil of the picture-taking lens to the film surface differs between the lenses, there occurs a difference in the output of the exposure meter. The reason is that if the position of the exit pupil is varied, the solid angle at which the light receiving element is subtended by the exit pupil is varied in spite of the aperture value remaining unchanged. More specifically, in FIGS. 4(a) and 4(b), reference character 8 designates the exit pupil of a long-focal-length lens, and 8a and 8b designate the upper and the lower ends of the lens, respectively. Designated by 9 is the exit pupil of a short-focal-length lens, and 9a and 9b designate the upper and the lower ends of such lens, respectively. The exit pupils 8 and 9 have the same aperture value, as can be seen from the fact that their upper ends 8a, 9a and the center 2' of the picture plane of the film 2 lie on the same straight line 10 and their lower ends 8b, 9b and the center 2' of the picture plane of the film 2 lie on the same straight line 11.
In FIG. 4(a), the exit pupils 8 and 9 have the same solid angle with respect to the center 2' of the picture plane, but the exit pupil 9 has a greater solid angle than the exit pupil 8 with respect to the center point 6a of the light receiving element 6 which lies short of the film surface. Therefore, the light from the exit pupil 9 contributes more to an increased intensity of illumination on the center point 6a of the light receiving element 6 than the light from the exit pupil 8. This also holds true of the upper and lower ends 6b and 6c of the light receiving element in FIG. 4(b). Accordingly, the quantity of light received by the entire light receiving element 6 differs between the short-focal-length lens and the long-focal-length lens and thus, even if the lenses have the same aperture value, the short-focal-length lens results in a greater output of the exposure meter. This is particularly important to open metering. In other words, the output of the exposure meter presents irregularity with respect to lenses of various focal lengths and various open F-values, as shown at a in FIG. 5, and such irregularity results in an error of exposure.
Therefore, if open metering is effected even with interchangeable lenses of the same open F-values, the exposure meter will produce different outputs for the short-focal-length lens and the long-focal-length lens and, if photography is carried out in accordance with the indications by the exposure meter, the resultant photographs will be improper in exposure.
In an attempt to eliminate these disadvantages, there have heretofore been proposed various methods, which may generally be classified into the following two types. One of them is a method of controlling the quantity of light entering the light receiving element and the other is a method of correcting the excessively great or small output of the light receiving element by mechanical or electrical means (often by both means). To carry out the former method, there is an apparatus which employs a cylindrical or a spiral shield disposed immediately in front of the light receiving element, but such a construction leads to an increased number of parts. This also means an increased weight of movable parts in the design wherein the light receiving element is retracted out of the picture-taking light path during photography, and such increased weight forms a burden to the mechanism. In the latter method, a signal pin for transmitting aperture values and focal lengths is provided on the lens side and the information from such signal pin is received by the camera body side to correct the output of the exposure meter, and this leads to an increased number of parts and complication of the entire construction.